This invention pertains to airfoils with control surfaces, and more particularly to means of coupling together a segmented control surface which also provides external visual indication of an actuator system failure.
Historically, in airfoil designs having a trailing control surface, e.g. rudders and elevators, it has been necessary to provide means to restrain the control surface in the event the means used to actuate the surface fails in a fashion which would allow the surface to rotate freely about its hinge-line. Some means is necessary to stabilize the failed control surface about its trail position relative to the airfoil so as to prevent surface flutter after the failure. Classically, mass was added at the proper location on the control surface so as to behave as a counterbalance, stabilizing the surface against flutter about its trail position. In a true flutter condition, the control surface is excited at its natural frequency and there is insufficient dampening to attenuate the vibration. The surface will continue to oscillate at greater amplitudes until failure occurs. Typically, this happens as a result of a complex coupling of the control surface with another structure, such as the wing. Counterbalancing the control surface with weight produces an overbalancing moment which increases as the motion of the wing or main airfoil increases and acts in the opposite direction, i.e., out of phase. Also, the overbalancing moment is directly proportional to the acceleration, which is not true if the biasing means is a spring as opposed to a mass.
Introduction of the fully powered control surface changed things somewhat. The full powered system not only provides adequate muscle to drive the control surface, but it provides a stiff locking means between the main airfoil and the control surface so that they act as a single unit which prevents the control surface from fluttering. However, in full power systems, safety requires three actuators, two of which are redundant, to provide adequate safety. For example, a failure that demands a hard over-control surface, like a valve jam, is passive in nature and can go undetected for long periods of time in the three-actuator system. A hard over-command on the first actuator requires a similar hard over-command in the opposite direction to neutralize the force of the first actuator. Since these two forces are equal and opposite, if there were only two actuators, the surface would return to a neutral or faired position only if there was a restoring force sufficient to increase the pressure in the jammed actuator above its relief valve setting. To remedy this problem a third actuator is employed.
New aircraft designs are placing great emphasis on fuel efficiency because of recent large fuel cost increases. One of the major factors in the efficiency of an aircraft is its weight. Weight reduction has caused the elimination of split or segmented elevators and rudder surfaces by employing single control surfaces to reduce the number of actuators required. Because of the hard-over actuator failure, discussed above, each of the three valve/actuator packages is totally separate to provide adequate redundancy so that the correctly controlled actuator can fight and overcome any incorrectly controlled unit. Where multiple actuators are employed on a common surface it is extremely difficult to prevent and inspect for a force fight between drives. Force fights introduce high fatigue cycling on structural members thereby increasing aircraft weight as well as creating a dead band around valve null, causing poor surface positionability.
Flight control system architecture is further complicated by the airworthiness requirements dictated by the Federal Aviation Regulations. Among these requirements is a requirement that the aircraft withstand a noncontained engine failure, which means that the flying debris as a result of the failure wipes out everything in its path, and still maintain flight control authority. This analysis is affected by the number of engines and the number of power systems as well as the distribution route for the power systems. An increase in the number of engines and in the number of power systems eases the effort to meet this requirement.
Further, a one-element control surface driven by three separate valve/actuator packages experiences force fights as discussed above. Because these force fights impose high loads on the surface, the three actuators need be close together to avoid racking or twisting the surface because of the high loads imposed where the actuators are separated over the span of the surface. Because of this location requirement and the high loads on the supporting structure, the actuator system must be constructed more ruggedly and hence heavier. Due to these complicated variables, aircraft with more than two engines may not necessarily need to achieve all the objectives or benefits of this invention.
It is an object of this invention to produce flight control systems which meet the necessary airworthiness requirements at minimum weight and cost. It is a further object of this invention to minimize the force fight between multiple actuators actuating a flight control surface and to devise a system which allows detection of any force fight readily and easily on the ground. It is a still further object of the invention to alleviate the need for the flutter control balance weights.